Generate letters to represent number using ruby?

I would like to generate a sequence of letters i.e. "A", "DE" "GJE", etc. that correspond to a number. The first 26 are pretty easy so 3 returns "C", 26 returns "Z", and 27 would return "AA", 28 "AB", and so on.

The thing I can't quite figure out is how to do this so it will handle any number passed in. So if I pass in 4123 I should get back some combination of 3 letters since (26 * 26 * 26) allows for up to +17,000 combinations.